In view of global environmental protection in recent years, there have been discussions with regard to switching refrigerants used for air conditioning from currently-used refrigerants with a high global warming potential (GWP), such as an R410A refrigerant, an R407c refrigerant, and an R134a refrigerant, to refrigerants with a low GWP, such as a carbon-dioxide refrigerant, an ammonia refrigerant, a hydrocarbon-based refrigerant, an HFO-based refrigerant, and an R32 refrigerant. Among these low-GWP refrigerants, an R32 refrigerant has an evaporating pressure and a condensing pressure that are substantially the same as those of an R410A refrigerant, and the refrigeration capacity per unit volume is greater than that of the R410A refrigerant, thus allowing for a compact apparatus. Therefore, the R32 refrigerant or a refrigerant mixture containing, for example, an HFO refrigerant and the R32 refrigerant as a main component are favorable candidates.
However, the density of the R32 refrigerant at the suction side of a compressor is smaller than that of the R410A refrigerant, thus causing the discharge temperature of the compressor to increase. For example, when the evaporating temperature is 5 degrees C., the condensing temperature is 45 degrees C., and the degree of superheat of the refrigerant when being suctioned into the compressor is 1 degree C., the discharge temperature of the R32 refrigerant increases by about 20 degrees C., as compared with that of the R410A refrigerant. An upper limit for the discharge temperature is fixed in accordance with the guaranteed temperature of refrigerating machine oil and a seal material of the compressor. Therefore, when the R32 refrigerant or the refrigerant mixture containing, for example, the HFO refrigerant and the R32 refrigerant as a main component is used, it is necessary to provide means for reducing the discharge temperature.
Furthermore, among large-size air-conditioning apparatuses (with a rated cooling capacity of, for example, about 20 kW or greater) generally used for air-conditioning buildings, there is a type of air-conditioning apparatus that has a plurality of indoor units connected to a single outdoor unit and that can perform a cooling operation, a heating operation, and a cooling and heating mixed operation. Specifically, in the cooling operation, the indoor units only perform cooling. In the heating operation, the indoor units only perform heating. In the cooling and heating mixed operation, the indoor units perform cooling and heating in a mixed fashion at the same time. In such a large-size air-conditioning apparatus, a low-pressure-shell-type compressor having an oil reservoir, a motor, and the like are provided at the low pressure side thereof is used as the compressor so as to reduce the amount of heat radiated from the compressor and to ensure the pressure resistibility of the compressor shell. However, such a low-pressure-shell-type compressor is different from a high-pressure-shell-type compressor in that, since a liquid refrigerant is separated at the oil reservoir when the refrigerant is to be suctioned into the compressor, there is a limit to reducing the discharge temperature even if the refrigerant to be suctioned is moistened.
Thus, an air-conditioning apparatus having a refrigerant circuit that reduces the discharge temperature of a compressor by injecting a refrigerant into the compressor so as to allow for a stable (highly-reliable) operation of the compressor has been proposed (e.g. see Patent Literature 1).